A bad slave/master cylinder will not result in the clutch slipping, but quite the opposite. If there's fluid missing, there has to be a leak somewhere.
As suggested, check your fuse box. It's the most common place for the fluid to go when the master leaks, and if it's there, you need to pull all the fuses out, and douse it in electrical cleaner. If you don't you'll be chasing electrical gremlins for the rest of the truck's life, or until the box is replaced.
What year/engine your truck is will help, A LOT.